1. Field of the Invention
This invention relates to a high-frequency device, and more particularly to a microwave filter and a high-frequency device related to the microwave filter.
2. Description of the Related Art
A communication apparatus for communicating information by wireless or by wire is composed of various devices, including amplifiers, mixers, and filters. That is, it includes many devices making use of resonance characteristics. For instance, a filter is composed of a plurality of resonating elements arranged side by side and has the function of allowing only a specific frequency band to pass through. Such a filter is required to have a low insertion loss and permit only the desired band to pass through. To meet these requirements, resonating elements with high unloaded Q values are needed.
One method of realizing a resonating element with a high unloaded Q value is to use a superconductor as a conductor constituting a resonating element and further use a material whose dielectric loss factor is very small, such as Al2O3, MgO, or LaAlO3, as a substrate. In this case, however, the unloaded Q value is 10,000 or more and the resonance characteristic is very sharp. As a result, the desired characteristic cannot be obtained unless the resonance characteristic is adjusted with high accuracy in the design stage.
To overcome such a problem, a resonator and a filter which have the function of adjusting the resonance frequency have been proposed. Methods of tuning the frequency of a resonator or a filter include a method of providing a dielectric whose permittivity depends on the applied electric field in the vicinity of a resonating element and thereby applying a voltage to the dielectric and a method of providing a magnetic material whose permeability varies with the applied magnetic field in the vicinity of a resonating element and applying a magnetic field to the magnetic material.
For example, what has been described in reference 1 (“Electrically tunable coplanar transmission line resonators using YBa2Cu3O7−x/SrTiO3 bilayers” by A. T. Findikoglu et al., Appl. Phys. Lett., Vol. 66, p. 3674, 1995) is a method of forming a coplanar resonator composed of an oxide superconductor film on an LaAlO3 substrate whose surface is covered with a dielectric SrTiO3 film whose permittivity depends on the applied electric field and applying a voltage between the central transmission line and the ground on both sides and thereby tuning the resonance frequency f. In this case, the tuning width Δf/f is 4%. Since a dielectric whose permittivity depends on the field strength, such as SrTiO3, has a high dielectric loss factor (tan δ), the unloaded Q value decreases to about 200. This causes the following problem: the advantage that use of a very low loss superconductor increases the unloaded Q value disappears.
Similarly, in reference 2 (“Tunable and adaptive bandpass filter using a nonlinear dielectric thin film of SiTiO3” by A. T. Findkoglu et al., Appl. Phys. Lett., Vol. 68, p. 1651, 1996), a tunable band-pass filter composed of a plurality of coplanar resonators capable of performing the aforementioned frequency tuning has been described. In this case, since the unloaded Q value of each resonator constituting the filter is small as described above, the rising and falling of the frequency passband called the skirt characteristics are gentle, impairing the frequency selectivity. There is another problem: when the frequency passband is changed by the application of a voltage, the insertion loss, skirt characteristics, and ripples in the frequency passband vary.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 9-307307 or Jpn. Pat. Appln. KOKAI Publication No. 10-51204 has disclosed a filter where a dielectric whose permittivity depends on a voltage is provided on a filter element and a pair of voltage applying electrodes is provided near the dielectric. In this case, it is possible to change the permittivity locally or distribute the permittivity according to the arrangement of electrodes or the applied voltage. This alleviates the above problem to some degree, that is, the problem of changes in the insertion loss, skirt characteristics, and ripples incidental to the tuning of the passing frequency band of the band-pass filter.
This method, however, requires not only a dielectric whose permittivity varies with the applied voltage but also voltage applying electrodes, leading to an additional loss caused by the electrodes. As a result, the unloaded Q value of a single resonator is as small as several hundred or less, which makes it impossible to obtain a filter with a sharp skirt characteristic.
Furthermore, when the tuning of the frequency is done by applying a voltage to the electrode pair and changing the permittivity of the dielectric uniformly, the loss due to the dielectric is great and in addition varies with the applied voltage. Consequently, the Q value of the resonating element constituting the filter varies as a result of tuning, which causes a problem: the insertion loss of the filter and the characteristics in the passband deviate from the desired characteristics. Moreover, this method permits the permittivity and dielectric loss factor to follow a spatial distribution and therefore cannot cause them to vary uniformly all over the surface.
Another method has been described in, for example, reference 3 (“Tunable Superconducting Resonators Using Ferrite Substrates” by D. E. Oates and G. F. Diome, IEEE MTT-S digest, p. 303, 1997). In this method, a plate of magnetic material Y3Fe5O12 (YIG) whose permeability varies with the applied magnetic field is provided on a microstrip-structure resonator formed on a substrate. A direct-current magnetic field is externally applied to the plate, thereby tuning the resonance frequency. Although the tuning width Δf/f is 3%, almost the same as that in the aforementioned dielectric control method, the unloaded Q value has been improved and is about ten times as large as that of a dielectric-control-type resonator. However, when a plurality of resonators with such a tuning function are arranged side by side, thereby forming a band-pass filter capable of tuning the passing frequency band, the electromagnetic coupling between the resonating elements and between the resonating elements and the input and output lines varies because the passing frequency band varies according to the application of the magnetic field. This variation causes a problem: the insertion loss, skirt characteristics, and ripple characteristics of the filter deviate from the original design. Moreover, when the passing frequency band is 5 GHz or less, the insertion loss becomes greater because of the magnetic loss.
Still another method has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-199024. In this method, a superconductive resonator is such that a vertically movable conductor rod, dielectric strip, or magnetic material rod is provided on a resonator with a single resonating conductor and the resonance frequency can be adjusted by controlling the position of the rod. However, to apply the method to a filter where a plurality of resonating elements are arranged side by side, it is necessary to move the conductor rod or the like on each resonating element over the same distance with high accuracy. There is another problem: changing the frequency leads to changes in the characteristics within the band, such as ripples or bandwidth.
In the description of reference 4 (“On the Development of Superconducting Microstrip Filters for Mobile Communications Applications” by Jia-Sheng Hong et al., IEEE Trans. Microwave Theory and Techniques, Vol. 47, No. 9, p.1656, 1999), a filter has been housed in a package and many tuning screws have been provided on the resonating elements and between the resonating elements. The screws are made to go down or up, thereby tuning the frequency. In this case, an increase in the loss as a result of the addition of the tuning function is smaller than in the aforementioned dielectric voltage applying method or magnetic material magnetic field applying method. However, since each screw has a different effect on the filter characteristics, the control of each screw must be performed independently and precisely. The optimum position of each screw must be made different according to the pattern of the filter. For this reason, this method has the problem of having many control parameters, being difficult to adjust, and being complex in structure.
On the other hand, in a communication system, such a skirt characteristic of a band-pass filter as prevents interference between adjacent frequency bands is required. Furthermore, a band-pass filter with a sharp skirt characteristic for making effective use of frequencies is needed.
When the skirt characteristic on the low-frequency side of the passband is made sharper, a filter circuit composed of a hairpin-type resonating element having a pole on the low-frequency side of the passband can be used as described in, for example, “1.5-GHz Band-Pass Microstrip Filters Fabricated Using EuBaCuO Superconducting Films” by Yasuhiro Nagai et al., Japanese Journal of Applied Physics, Vol. 32, p. L260, 1993.
Conversely, when the skirt characteristic on the high-frequency side of the passband is made sharper, a forward-coupled filter having a pole on the high-frequency side of the passband can be used as described in, for example, “Compact Forward-Coupled Superconducting Microstrip Filters for Cellular Communication” by Dawei Zhang et al, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, Vol. 5, No. 2, p. 2656, 1995.
Furthermore, when both sides of the passband are made sharper, a quasi-elliptic-function-type filter having poles on both sides of the passband can be used as described in, for example, “On The Performance of HTS Microstrip Quasi-Elliptic Function Filters for Mobile Communications” by Jia-Sheng Hong et al., IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 48, No. 7, p. 1240, 2000.
In any of the above cases, use of multiple stages of resonating elements enables the skirt characteristics to be made sharper. Since metal filters or dielectric filters cause great losses, they cannot be made multistage. However, use of superconductive filters using superconductors as resonating elements makes it possible to realize multiple stages of filters.
When a communication system requires a very sharp skirt characteristic, even if the filter has poles, a great many resonating elements must be used to realize a multistage structure, which makes the filter circuit larger. For this reason, to produce such a large filter circuit, a very large substrate is needed.
However, it is difficult to produce such a large substrate by using Al2O3 (sapphire), MgO, LaAlO3, or the like, used for a microstrip-line-type superconductive filter, which results in an increase in its production cost. It is also difficult to form a superconductor film on a large substrate. That is, when a band-pass filter with a very sharp skirt characteristic required in a communication system is realized using conventional techniques, the following problems are encountered: one problem is that it is difficult to prepare a large substrate on which a superconductor film has been formed; and another problem is that, even if such a substrate has been prepared, the production cost is very high.
Furthermore, a superconductive band-pass filter with a high-power-resistant transmission characteristic, such as a transmission filter in a wireless base station, is realized by constructing the filter using large resonating elements as described in, for example “Elliptic-Disc Filters of High-Tc Superconducting Films for Power-Handling Capability Over 100 W” by Kentaro Setsune et al., IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 48, No. 7, p.1256, 2000. However, to realize a sharp skirt characteristic required in the system, it is necessary to use a large number of resonating elements for a multistage structure. This causes the following problems: it is difficult to prepare such a large substrate that enables a lot of large resonating elements to be formed; and if such a substrate has been prepared, its production cost is very high.
There arises another problem: when a superconductive filter circuit becomes large, this makes larger the mounting system that houses the filter circuit, resulting in an increase in the cooling cost for realizing the superconducting characteristics.
On the other hand, a band-pass filter whose characteristics, including the center frequency and bandwidth, are variable is indispensable to the construction of a communication infrastructure capable of flexibly copying with modifications to the system. With a conventional characteristic-variable band-pass filter, each amount of the coupling between resonating elements constituting the filter and the external Q were controlled independently, thereby obtaining the desired filter characteristic and its change as described in Jpn. Pat. Appln. KOKAI Publication No. 9-307307. Therefore, to change the characteristic of a multistage filter with a sharp skirt characteristic by the method of the conventional characteristic-variable band-pass filter, it is necessary to control a great many couplings between resonating elements, resulting in an enormous number of parameters to be controlled, which makes it difficult to change the characteristic of the multistage filter.
As described above, it was not easy to obtain a band-pass filter with a sharp skirt characteristic because a large substrate was needed in the prior art. It was also difficult to adjust the transmission characteristic of the filter accurately. For this reason, there have been demands toward realizing a filter device which has a sharp skirt characteristic and is capable of obtaining a desired transmission characteristic easily.